Attendants on Cathay Pacific's direct flights to New York say they are being limited to around two trips a month on the route because of concerns about their exposure to cosmic radiation.

But no warnings are being given to frequent fliers on the transpolar route because the Civil Aviation Department (CAD) says passengers do not fly enough to be at risk.

Unions representing both pilots and flight attendants believe passengers should be given more information about cosmic radiation, which has been linked to marginal increases in the risk of developing cancer.

'The public ought to be aware that there are possible dangers,' said John Findlay, general secretary of the Aircrew Officers Association.

Becky Kwan Siu-wa, head of the Flight Attendants Union, said frequent travellers should be warned.

Fliers are exposed to increased doses of cosmic radiation at altitudes above 8,000 metres over the north and south poles, where the atmosphere is thinner.

Concern over radiation exposure coincided with the launch of the daily Cathay Pacific transpolar direct flights to New York last July.

Cathay has introduced a stringent radiation monitoring programme, which allows pilots and flight attendants to know how much radiation they have been exposed to and to ensure it never exceeds safe limits.

Data on radiation levels on each flight is processed so pilots or flight attendants can check their exposure. Rosters are tailored to keep them within safe limits.

Ms Kwan said a formula had been worked out which effectively limited flight attendants to about two round trips a month on the polar route.

'If you do two and a half polar flights a month, you are in the danger zone,' she said. 'At first, when we heard about this, everybody was worried. But we have had regular meetings with the CAD and Cathay and guidance from an aviation doctor.'

She said the airline has ensured no flight attendant registered radiation levels above what are regarded as safe limits.

But Ms Kwan said passengers should be given more information about the risks.

'I think the same message should be communicated to the travelling public,' she said. 'If you are a frequent traveller you shouldn't do more than so many trips on this route.'

Mr Findlay said he believed Cathay Pacific had done as much as any other airline to keep staff informed about cosmic radiation and to monitor their exposure to it and he was happy with its response. But he said regulatory bodies such as CAD and the International Civil Aviation Organisation should be giving out more information on the issue.

Mr Findlay said he believed that in future, regular warnings about cosmic radiation might be given to airline crews and the public.

'It is early days yet [in terms of scientific research],' he said. 'As time goes on, people will become more and more concerned.'

A spokeswoman for Cathay Pacific said there was no quota on the number of New York flights cabin crew could serve on but she said rosters would be adjusted if cumulative radiation readings for staff approached the top end of safety limits.

Cathay has a passenger information section on its website that says the increased cancer risks brought on by exposure to cosmic radiation is minimal and one 'most people would probably not consider unacceptable'.

A person flying direct from Hong Kong to New York every two weeks for 20 years will increase their risk of death from cancer from 23 per cent to between 23.11 and 23.14 per cent, it says.

A CAD spokeswoman said a cosmic radiation monitoring programme for pilots and flight attendants had been introduced in August 2002 'in accordance with the International Civil Aviation Organisation requirements and international practices'.

She said the organisation does not require similar monitoring of passengers, as they are less exposed to cosmic radiation than crew members.